Speaker: Xiang Cheng, University of Minnesota, Department of Chemical Engineering and Material Science

Subject: Impact response of granular materials: From the origin of the universe to catastrophic asteroid strikes

Granular materials are large conglomerations of discrete macroscopic particles. Examples include seeds, sand, coals, powder of pharmacy, etc. Though simple, they show unique properties different from other familiar forms of matter. The unusual behaviors of granular materials are clearly illustrated in various impact processes, where the impact-induced fast deformation of granular materials leads to emergent flow patterns revealing distinctive granular physics. Here, we explored the impact response of granular materials in two specific experiments:

First, we performed the granular analog to “water bell” experiments. When a wide jet of granular material impacts on a fixed cylindrical target, it deforms into a sharply-defined sheet or cone with a shape mimicking a liquid of zero surface tension. The jets' particulate nature appears when the number of particles in the beam cross-section is decreased: the emerging structures broaden, gradually disintegrating into diffuse sprays. The experiment reveals a universal fluid structure arising from the collision of discrete particles, which has a counterpart in the behavior of quark-gluon plasmas created by colliding heavy ions at the Relativistic Heavy Ion Colliders.

Second, we investigated impact cratering in granular media induced by the strike of liquid drops—a ubiquitous phenomenon relevant to many important environmental, agricultural and industrial processes. Surprisingly, we found that granular impact cratering by liquid drops follows the same energy scaling and reproduces the same crater morphology as that of asteroid impact craters. Inspired by this similarity, we develop a simple model that quantitatively describes various features of liquid-drop imprints in granular media. Our study sheds light on the mechanisms governing raindrop impacts on granular surfaces and reveals an interesting analogy between familiar phenomena of raining and catastrophic asteroid strikes.